Thin-walled structures often require access ports or other openings through the basic structure. Such openings perform functions such as permitting the passage of tubing, providing visual or instrumentation inspection access, and mounting of operating elements to the thin-walled structure. An example of interest is the mounting of fuel nozzles and passage of the fuel lines to the fuel nozzles in a stator casing of a gas turbine engine.
The thickness of the thin-walled structure is usually selected to be no greater than necessary to carry its normal loadings at locations away from the opening, particularly in weight-sensitive applications such as a gas turbine engine. However, whenever there is such an opening in a thin-walled structure that must carry a significant applied loading during service, there is a stress concentration in the vicinity of the opening. To reduce loads at the opening, the thin-walled structure is typically thickened locally in the neighborhood of the opening. This local thickening is termed an “embossment”. Embossments may also be formed at locations that are to be hardened, such as attachment points that are used to attach the thin-walled structure to other structure.
The embossment may be created in any of several ways. In one approach that is often used where the thin-walled structure is cylindrically symmetric and there are a number of openings arranged around the cylindrical circumference, an extra thickness of metal is cast or formed into the thin-walled structure around the cylindrical circumference. The excess portion of the extra thickness of material, at locations away from the locations where the openings are to be provided, is machined away. The removal of the excess material is usually costly, due to the machining expense and to the cost of the material that is machined away.
In another approach, the embossment material may be provided as a discrete piece and then joined to the thin-walled structure at the appropriate location. This technique requires the use of conventional fusion welding, inertial welding, diffusion bonding, or some other attachment technique. The attachment technique must be relatively inexpensive to employ, and must allow the joining of the embossment material of the desired shape and size. Fusion welding results in a cast interface between the thin-walled structure and the embossment that exhibits a reduced strength that is usually unacceptable in high-strength, weight-critical applications. Fusion welding is also prone to microfissure formation in difficult-to-weld alloys such as Rene™ 41 and Waspaloy. Solid state welding techniques such as diffusion bonding provide superior mechanical properties without microcracking, as the joint is not formed by solidification of molten metal. The resulting tensile strength and fatigue behavior are generally equal to or less than those of the parent metal. Solid state diffusion welding techniques, however, are generally rather slow and uneconomical to utilize for many applications. The use of inertial welding, another solid-state welding technique, limits the ability to create embossments of arbitrary shapes that have the proper orientation relative to the part axis, a significant shortcoming in many specialty applications.
There is a need for an improved approach to the fabrication of articles with embossments, particularly thin-walled structures that are used in weight-critical applications. The present invention fulfills this need, and further provides related advantages.